bpf, arm64: remove structs on stack constraint

While introducing support for 9+ arguments for tracing programs on
ARM64, commit 9014cf56 ("bpf, arm64: Support up to 12 function
arguments") has also introduced a constraint preventing BPF trampolines
from being generated if the target function consumes a struct argument
passed on stack, because of uncertainties around the exact struct
location: if the struct has been marked as packed or with a custom
alignment, this info is not reflected in BTF data, and so generated
tracing trampolines could read the target function arguments at wrong
offsets.

This issue is not specific to ARM64: there has been an attempt (see [1])
to bring the same constraint to other architectures JIT compilers. But
discussions following this attempt led to the move of this constraint
out of the kernel (see [2]): instead of preventing the kernel from
generating trampolines for those functions consuming structs on stack,
it is simpler to just make sure that those functions with uncertain
struct arguments location are not encoded in BTF information, and so
that one can not even attempt to attach a tracing program to such
function. The task is then deferred to pahole (see [3]).

Now that the constraint is handled by pahole, remove it from the arm64
JIT compiler to keep it simple.

[1] https://lore.kernel.org/bpf/20250613-deny_trampoline_structs_on_stack-v1-0-5be9211768c3@bootlin.com/
[2] https://lore.kernel.org/bpf/CAADnVQ+sj9XhscN9PdmTzjVa7Eif21noAUH3y1K6x5bWcL-5pg@mail.gmail.com/
[3] https://lore.kernel.org/bpf/20250707-btf_skip_structs_on_stack-v3-0-29569e086c12@bootlin.com/



Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20250709-arm64_relax_jit_comp-v1-1-3850fe189092@bootlin.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>